To improve the fuel economy of automobiles, there is an outstanding demand for a reduction in the idling drag of wet-type multiplate clutches for use in automatic transmissions or the like. With a view to meeting this demand, it has been a conventional practice to form spiral grooves, outer-end-closed grooves or the like on the surfaces of friction linings in a clutch such that under a centrifugal force, a force which pulls off a counterpart member is produced at the surface of each friction lining to reduce a drag torque in a stable state during idling.
FIG. 1 is a cross-sectional view illustrating the fundamental construction of a wet-type multiplate clutch. Separator plates 3 are connected through splines to spline grooves 51 on a drum 5, while friction plates 4 which are arranged opposite the separator plates 3 are connected through splines to spline grooves 82 on a hub 8. Upon engagement of the clutch, a piston 9 is pressed under a hydraulic pressure toward the right-hand side as viewed in the drawing so that the separator plates 3 and friction plates 4 are pressed as engagement members against a flange 2 and a snap ring 1. As a consequence, a torque is transmitted from the drum 5 to the hub 8. Designated at numeral 81 are lubrication holes arranged in the hub 8.
The friction plates 4 are each provided with a core plate 41 in the form of a metal disc and friction linings 7 fixed on opposite sides of the core plate 41, respectively. As mentioned above, the friction linings are each provided on a surface thereof with various oil passages or oil grooves so that owing to the production of a hydraulic pressure under a centrifugal force, forces which press the friction surfaces act to achieve a reduction in drag torque upon idling and a prompt release upon disengagement of the clutch.
With friction plates of the conventional construction, a large oil amount leads to a reduction in the coefficient of friction under the influence of pressure-producing grooves upon engagement of a clutch so that, while the engaging torque remains small, the engagement progresses and the torque abruptly increases at the end of braking time. Such a large oil amount, therefore, becomes a cause of a shift shock. It may, therefore, be contemplated to make the pressure-producing grooves smaller in an attempt to obtain adequate friction characteristics. This, however, leads to a reduction in the force at friction surfaces, said force serving to pull off the counterpart members during idling, thereby bringing about an increase in idling drag.
Substantial difficulties have been encountered in designing grooves such that they can achieve both a reduction in idling drag and adequate friction characteristics.